A series resonant converter using series resonance of an inductance of a resonance inductor and a capacitor of a resonant capacitor is commonly used as a converter with high power conversion efficiency. A series resonant converter is mainly divided into a current mode series resonant converter in which a resonant capacitor is connected in series with a primary winding or a secondary winding of a transformer and a voltage mode series resonant converter in which a resonant capacitor is connected in parallel with a primary winding or a secondary winding of a transformer. A current type series resonant converter has been disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2003-324956. A voltage type series resonant converter has been disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2003-153532.
Such a series resonant converter can reduce switching losses of a switching device by zero current switching (ZCS) in which a switching device performs switching when a current flowing in the switching device is near zero and by the switching device performing switching in a lagging current mode. However, in such a series resonant converter, it has been suggested that there is power loss caused by energy of a resonant circuit returning to the DC power supply via a feedback diode connected in parallel with the switching device. In other words, a feedback current to the DC power supply improves power efficiency in that it returns the energy to the DC power supply, but the energy supplied from the DC power supply to the resonant circuit is returned to the DC current, which causes unnecessary loss of circuit caused by the current which has not been supplied to a load device.
Furthermore, in the series resonant converter such as that described above, specifically, when a voltage charged in a resonant capacitor by series resonance is higher than that of the DC current, the energy accumulated in the resonant capacitor, accompanied with turning off one switching device which was turned on, becomes current that flows to a feedback diode connected in parallel with the switching device which was turned on. Therefore, at the moment when the other switching device turns on, reverse voltage is applied to the feedback diode flowing feedback current, and precipitous recovery current (reverse recovery current) flows in a recovering time until a reverse direction blocking characteristics of the feedback diode is recovered, which generates power loss and noise.
In addition, a technology with respect to a voltage type series resonant converter disclosed in Japanese Unexamined Patent Application Publication No. 2006-191766 is as follows. The voltage type series resonant converter uses resonance of a resonant inductor and a resonant capacitor connected in parallel with a primary side of a transformer, and the resonant capacitor is charged more than a DC power supply voltage. A charge of the resonant capacitor is returned to a DC power supply and recovery current flows to a feedback diode. In this circuit, two capacitors are connected in parallel with each of the two diodes of a bridge rectifier circuit that is composed of four diodes. However, these capacitors are not selected so as to resonate in series with a resonant inductor. This voltage type series resonant converter realizes different output characteristics in which a rectifier circuit functions as a voltage-doubler rectifier circuit when output current is small and as a bridge rectifier circuit when output current is large.
Thus, in a conventional series resonant converter, in every half-cycle time of a switching cycle of a switching device, energy corresponding to resonant energy flows in a feedback diode as a large feedback current. Therefore, current which is not supplied to a load device becomes large, which generates unnecessary circuit loss.
In addition, in every half-cycle time of a switching cycle of a switching device, since recovery current flowing in a reverse direction is generated at a feedback diode, not only power loss of a feedback diode is increased, but also unnecessary turn-on power loss is generated in a switching device in an amount as much as the recovery current flowing, thereby reducing power efficiency of a resonant converter. Moreover, since such recovery current has a sharp waveform, it has a deficiency in generating noise even if its current waveform is rendered to be a sine wave form by resonance.